1. Field of the Invention
The present invention relates to a bar-shaped light guide adapted to allow the rays of light incident from one end face to be emitted from an emitting surface provided along the longitudinal direction, an illumination unit combining the light guide with a light emitting source, and an image-scanning device in which the illumination unit is incorporated.
2. Description of the Background Art
A bar-shaped light guide used as part of an image-scanning device such as a facsimile machine, a copying machine and a scanning device often has a light-emitting unit such as a LED disposed only on one of its ends. In the case where the light-emitting unit is disposed only on one end of the bar-shaped light guide, the number of light-emitting units is reduced and this is advantageous in terms of heat generation and costs. However, it is necessary to allow the light to be evenly emitted from an emitting surface without decreasing the intensity of illumination.
For example, Patent Document 1 discloses a bar-shaped light guide in which an end face opposite to an incident end face is mirror-finished and a member with high reflection efficiency is disposed outside the end face on the opposite side. In this manner, the light incident from one end face side, traveling through the light guide and reaching the end face on the opposite side is caused to reflect toward the incident side. By repeating this, all the incident rays of light are completely consumed as the irradiated light from a light-scattering pattern.
Patent Document 2 discloses a bar-shaped light guide in which an end face opposite to an incident end face is used as is as a rough surface to reduce manufacturing costs, while a light-scattering pattern of a section close to this end face is made wide. In this manner, a large portion of incident rays of light is consumed as scattered light before reaching the end face on the opposite side.
Patent Document 3 also disclosed a bar-shaped light guide in which an end face opposite to an incident end face is coated with a white paint to scatter the light reaching the end face. Patent Document 4 is a technical field irrelevant to the bar-shaped light guide, but discloses a reflecting mirror of a corner-cube type for retro-reflection.
In the case of a line-illuminating device (i.e., an illumination unit) used as part of an image-scanning device such as a facsimile machine, a copying machine and a scanning device, a bar-shaped light guide is often housed within a white casing. To prevent the bar-shaped light guide from being removed from the casing after once being housed or from being displaced inside the casing, configurations disclosed in Patent Documents 2 and 3 are known.
In the configurations disclosed in Patent Document 2 (see FIG. 6) and Patent Document 3 (FIG. 4), a dimple section is formed on a side surface other than a side surface where a light-scattering pattern of the bar-shaped light guide is formed. A projection formed on the inner surface of the casing is fitted into this dimple section.
[Patent Document 1] Japanese Patent Application Publication No. 8-163320
[Patent Document 2] Japanese Patent Application Publication No. 10-126581
[Patent Document 3] Japanese Patent Application Publication No. 11-84544
[Patent Document 4] Japanese Patent No. 2954709
The light guide described above is housed within a white casing to expose an emitting surface and is incorporated within an image-scanning device. The light guide is made of an acrylic resin of which the degree of transparency is high, while the casing is made of a low-cost resin.
The image-scanning device such as a facsimile machine, a copying machine and a scanning device sometimes undergoes temperature increase during transport or storage. Accordingly, when the light guide is housed within the casing, it is necessary to take the thermal expansion difference into consideration. However, if a line-illuminating device is left unattended at a high temperature and then cooled, a gap is produced between an end face of the light guide, in particular the end face opposite to a side where a light-emitting unit is disposed, and an internal surface of the casing because the light guide has a larger degree of shrinkage than the casing due to the difference in material. Such a gap is easily produced by a manufacturing error and the like as well as the difference in the coefficient of thermal expansion.
In this manner, once the gap is produced, the light which has been reflected and returned to one end side in a condition in which any gap is produced does not reflect on the end face on the opposite side, but penetrates it and is scattered on the internal surface of the casing. The scattered light is emitted from the vicinity of the end face on the opposite side. In this manner, the illumination intensity becomes abnormally high in the vicinity of the end face on the opposite side as compared with the other areas. This tendency is remarkable when the end face opposite to the light-emitting unit is mirror-finished as disclosed in Patent Document 1.
The illumination efficiency becomes worse in the structures disclosed in Patent Documents 2 and 3.
FIG. 12 is a view showing a dimple section formed on a conventional light guide as shown in Patent Documents 2 and 3. The dimple section is made rectangular or trapezoidal and its side and bottom surfaces are made flat. As a result, when the rays of light incoming from the end face of the light guide which travel through the light guide strike the side or bottom surface, the rays are easily reflected in the same direction.
FIG. 13 is a graph comparing an illumination unit according to the present invention with a conventional illumination unit as to the relationship between a main scanning direction position and an optical power by each color (red, blue, and green). The line segments P1, P2 and P3 show the optical power of each color of the conventional light guide in which a dimple section is formed in a position of 120 mm in the main-scanning direction. As is obvious from this graph, when the dimple section of a rectangular shape or a trapezoidal shape is formed, the optical power drastically rises in the vicinity of 120 mm in the main scanning direction wherein unevenness of the illumination intensity is generated in the main-scanning direction.